Facebook's shares are trading at less than half their IPO price, but the social network is still valued at $40.61bn. Photograph: Mario Tama/Getty Images

Facebook's shares hit new lows on Monday, sinking to less than half their initial public offering price and halving the fortune of founder Mark Zuckerberg.

By mid-morning, Facebook's shares hit a new low of $18.75 (£11.91), less than half the $38 they were sold for in May amid the most heavily hyped stock sale in recent history.

The slump has knocked close to $10bn off the value of Zuckerberg's stake in the firm, which is now worth about $9.5bn.

Facebook's latest share slide comes after the expiration of a lock-up period that allows some of its earliest investors to sell more of their shares. The expiration increased the pool of available shares by 60% and confirmed analysts' fears that it might lead to more falls in Facebook's already battered share price.

Some of Silicon Valley's most prominent investors are among those now able to reduce their holdings. Details of which large shareholders have decided to cash in are not yet available.

Facebook's share price faces a series of other potential challenges in the next few months as more lock-up periods expire and staff are allowed to sell shares.

Facebook's fall comes as its peers, too, have faced investor scepticism. Shares in Groupon, the daily deals site, are also close to new lows and early investors including Marc Andreessen, one of Silicon Valley's most respected investors, have been cutting their holdings.

Shares in Zynga, the online games firm, have fallen more than 68% since last year's initial public offering. The firm, whose hits include Words With Friends and Draw Something, was responsible for 12% of Facebook's revenue last year.

On the day of its IPO in May Facebook was briefly valued at more than $100bn, more than the combined worth of Nike and Goldman Sachs. The social network is now valued at $40.61bn.

The company is on course to claim a billion people as users this year. But while its size and reach are undisputed analysts fear that the firm has been unable to find a way to make money from its mobile users, the fastest growing sector of its business.